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Citadel / Citadel Securities Interview Question Bank

Citadel and Citadel Securities run two distinct loops sharing one brand. Citsec (the market maker) hires for HFT software, quant research and quant dev; its phone screens lean on small, sharp coding prompts (merge-K, ring buffers, stock-DP ladders) and the loop is famously single-fail — any weak round and the next interview is cancelled within a day. Citadel (the hedge fund) and its sub-orgs (GQS, EQR, NXT) lean harder on probability brainteasers, deep resume drilling and a multi-week team-match stage where coding bar drops but cultural and PM-style fit dominates the offer decision.

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Best Time to Buy and Sell Stock — Full DP Ladder

Phone-screen staple at Citsec: start with the single-transaction max profit, ladder up through unlimited transactions, then bounded-k transactions, and on some loops add cooldown / transaction-fee variants. Interviewers grade as much on the generalization narrative (state definition, complexity, edge cases) as on the final code.

complexity-analysis

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